This invention relates to an electrical junction block used for the wiring of the electrical equipment of an automobile and a method for manufacturing the same.
For example, as shown in FIG. 1, in the electrical junction block of an automobile, metal bus bars 1 formed in preset circuit patterns are arranged on respective insulative plates 2, the insulative plates 2 are put upon others and set into a case formed of an upper case body 3A and a lower case body 3B, and functional parts such as fuses 4 and relays (not shown) are mounted thereon. The electrical junction block is connected to input-output electric wires via a connector 5. Further, a cover is mounted if required.
Since such an electrical junction block is restricted by the installation place thereof and the way for installing the same, it is required to design the same extremely efficiently and rationally. Particularly, in recent years, as the number of electrical devices of an automobile increases and the function thereof becomes greater, the circuit in the electrical junction block becomes significantly larger and the number of functional parts to be mounted becomes larger, so that the size of the electrical junction block will be inevitably made larger.
The importance for the manufacturing of the electrical junction block is to finish portions of bus bars, which are bent and form a tab, and wherein the curved and end portions of bus bars have high dimensional accuracy so as to enhance the easy assembling and the dimensional stability after the assembling. In particular, the dimensional accuracy for the tab portion must be high because it is formed to pass through the case body.
Various materials can be considered to be used for the material of the electrical junction block, but in general, a synthetic resin which is called an engineering plastic is used from the viewpoint of the working temperatures, formability and workability, economical efficiency and the like. Among the engineering plastics, nylon which is excellent in the heat resistance and the economical efficiency is frequently used since the ambient temperature of the electrical junction block used particularly in the engine compartment becomes equal to or higher than 70.degree. C. Nylon has advantages that it can be efficiently formed into a complicated configuration and it has a sufficiently high mechanical strength at high temperatures.
In contrast, nylon has a defect that it has a moisture absorption property and the dimension thereof significantly varies in the dried/moisture absorbed states. That is, a nylon article immediately after the molding is set in the dried state and is weak against mechanical shock, and it absorbs moisture with time and the dimension thereof varies.
Therefore, in the actual manufacturing process, a nylon article is subjected to a moisture absorbing process after molding until an equilibrium condition is attained and is then fed to the succeeding stage. The moisture absorbing process is generally effected by placing the molded product in an atmosphere with high humidity and at a high temperature for up to several tens of hours, but the quantity of work actually effected for the process is extremely large and the installation cost for effecting the process is extremely great. Further, a large variation in the dimension of the nylon article caused by moisture absorption means that the dimension of the nylon article tends to be changed even if it has been subjected to the moisture absorption process after molding, that is, the nylon article will absorb moisture and the dimension thereof will change when it is placed in a high temperature and a high himidity condition, and in contrast, the nylon article gives off moisture and the dimension thereof is changed when it is placed in a dried condition.
When the dimension of the case body of the electrical junction block is changed, the integration of the case body with the bus bars cannot be easily attained and the degree of matching between various portions will be lowered. In particular, since through holes are formed in a tub setting portions of the case body, it becomes impossible to insert the tabs into the through holes if a match in dimension between them cannot be attained. If the tab is forcedly inserted into the through hole, the bus bar will be bent or broken, thereby causing a short-circuit or breakage in the circuit. The same problems as described above will arise when a variation in dimension caused according to a dry/moisture absorption condition after assembling has occurred.
An influence due to the variation in dimension becomes larger as the electrical junction block becomes larger, and a variation in dimension can be suppressed within a tolerance if the length of one side thereof is equal to or less than 170 mm and no practical problem occurs. However, if the length is set to be larger than the above value, the variation in dimension cannot be suppressed within the tolerance, causing the assembling stability and the reliability of the electrical junction block to be remarkably lowered.